Lamp filament structure

ABSTRACT

A lamp filament is reliably and firmly connected both electrically and mechanically to a lead wire or mounting post by a body of metallic material intimately entrapping the filament and metallurgically bonded to the lead wire.

United States Patent Inventor Patented Assignee LAMP FILAMENT STRUCTURE 4 Claims, 4 Drawing Figs.

U.S. Cl 313/271, 313/331 Int. Cl H01j1/88, l-101j 19/42 Field of Search 313/271, 272,311, 331

[56] References Cited UNITED STATES PATENTS 3,119,172 1/1964 Mazenko et al. 29/628 FOREIGN PATENTS 1,062,008 3/1967 GreatBritain Primary Examiner-David Schonberg Assistant ExaminerPaul A. Sacher AttorneyMason, Kolehmainen, Rathburn & Wyss ABSTRACT: A lamp filament is reliably and firmly connected both electrically and mechanically to a lead wire or mounting post by a body of metallic material intimately entrapping the filament and metallurgically bonded to the lead wire.

PATENTEnnm 12 IHYI 3.612.940

F IG. 2

PRESSURE \Js I4 I J1 HEAT 0nd PRESSURE l6 FIG. 4 m I l4 IN VENTOR; RICHARD E. SIMS,

) 777M001 kolchmAjn v x-khbqr m Attorneys 1 LAMP FILAMENT STRUCTURE This application is a division of application Ser. No. 765,149 now US. Pat. No. 3,561,084 filed Oct. 4, 1968.

The present invention relates to a filament structure for incandescent lamps.

In the manufacture of incandescent lamps a filament such as a fine tungsten wire is mounted between a pair of lead wires or posts which serve both to support the filament and to complete a path for energization of the filament. Consequently it is of paramount importance that the interconnection between the filament and the lead wires or posts be firm, reliable, and characterized by extremely low electrical resistance.

In brief, in accordance with the present invention a lamp filament is mounted to a lead wire by a metallurgical or intermetallic bond created between a body of metal and the lead wire, the filament being entrapped within the body of metal.

The invention together with the above and other objects and advantages may be better understood from the following detailed description of a process for forming the structure of the invention, in the course of which description reference is made to the drawing wherein:

FIGS. 1-4 are somewhat diagrammatic and fragmentary illustrations of successive operations carried out in a process of forming a lamp filament structure in accordance with the present invention.

Having a reference now to the drawing, there are illustrated certain of the steps of a process performed in accordance with the present invention for mounting a lamp filament 10 to a lead wire 12 with a body 14 of metal. In general, and referring first to FIG. 1, the filament 10 is placed on the lead wire 12 and the body 14 of metal is positioned to overlie the filament 10 and lead wire 12. The metal body 14 is then moved toward the lead wire 12 to sandwich the filament 10 between the lead wire 12 and the body 14, as indicated in FIG. 2. The body 14 is heated to a plastic or semiplastic state, but not to a liquid state, while being pressed against the filament l and the lead wire 12. As a result the filament is intimately surrounded or entrapped in the metal of the body 14 while an intermetallic or metallurgical bond is formed by diffusion at the interface of the body 14 and the lead wire 12. Due to the extensive and firm mechanical contact between the filament 10 and the body 14, and the thermocompression diffusion bond between the body 14 and lead wire 12, a highly reliable and conductive connection is produced.

The present invention is particularly suited to the manufacture of miniature lamps such as numeric readout lamps wherein several filaments such as the filament 10 illustrated in the drawing may be connected between several lead wires or posts 12 and sealed in a vacuum beneath a transparent faceplate. In the illustrated embodiment of the invention, the filament 10 is a tungsten wire having a diameter in the neighborhood of 0.3 mils and wound in a spiral having a diameter in the neighborhood of l to 2 mils in accordance with known practice. The lead wire or post 12, only the end which is illustrated, may be of any desired size and configuration, and preferably comprises a conductor adapted to extend from the interior of the finished lamp for completing an electrical connection to the filament 10. In order to permit an intermetallic or metallurgical bond between the lead wire 12 and the body 14 of metal, the lead wire 12 has a surface formed of a metal capable of producing a diffusion bond with the metal the body 14.

In the preferred embodiment of the invention, the lead wire 12 is formed of a suitable conductive metal such as copper, dumet," or the like, while the body 14 is formed of gold. In order to permit the formation of a diffusion bond between the body 14 and the post 12, the portion of the post 12 to which the body 14 is bonded is provided with a coating of highly pure gold. In addition, in order to prevent diffusion between the body of the conductor 12 and the gold coating, an intermediate layer of nickel or other suitable material may be provided.

The body 14 of metal may be provided in any desired manner. In the illustrated arrangement, a metal wire 16 preferably fabricated of highly pure gold and having a diameter in the neighborhood of 1.5 mils is advanced from a supply such as a spool (not shown) through a tip member 18 having a central passageway 20 (FIG. 1) approximately 2 mils in diameter. The body 14 is initially in the form of a ball or bead having a diameter of about 6 to 8 mils formed at the end of the wire 15.

As illustrated in FIG. 1, the filament 10 is positioned upon the lead wire 12 between the lead wire and the body 14 of metal. In the preferred arrangement the filament 10 is located on the substantially flat end of the lead wire 12, although it should be appreciated that the filament 10 may be attached to other portions of the lead wire 12 if desired The arrangement shown in the drawing has the advantage that the connection created between the filament and the lead wire is small in size and low in profile and is therefore particularly suited for use in extremely small lamps.

After the body 14, filament l0, and lead wire 12 have been aligned with one another, the body 14 is moved toward the filament l0 and lead wire 12 as illustrated in FIG. 2. In the preferred method, the body 14 is pressed toward the lead wire 12 before heat is applied to the body 14. This operation serves to compress or crush the spiral wound filament 10 into close engagement with the surface of the lead wire 12 due to the relative rigidity of the metal of the body 14. As a result the filament wire takes up a minimum of space in the region of the connection to the lead wire. This pressing operation may conveniently be carried out by forcing the tip member 18 against the ball or beadlike body 14 thereby to force the body 14 toward the lead wire.

In accordance with an important aspect of the present invention, there is created a firm and reliable connection between the body 14 and the filament 10 and also between the body 14 and the lead wire 12. As indicated in FIG. 3, this is accomplished by subjecting the body 14 to heat and to pressure to the end that the body 14 entraps or intimately envelops the filament l0 and at the same time forms an intermetallic or metallurgical bond with the lead wire 12.

More specifically, the body 14 of metal is heated to a temperature sufficient to place the metal in a plastic or semiplastic state, but insufficient to melt the metal. The temperature to which the body is heated in accordance with the invention is high enough so that the metal exhibits a substantial increase in intrinsic malleability and so that diffusion takes place at the interface of the body 14 and the surface of the lead wire 12. However, the temperature is maintained below the melting point of the metal of the body 14 and also well below the temperature at which the material of the lamp filament 10 becomes brittle.

In the preferred embodiment of the present invention, the body 14 is formed of gold and is heated to a temperature in excess of about 450 C., this being the approximate temperature at which gold is capable of diffusion. Furthermore the temperature of the body 14 is maintained below the melting point of gold which is about l,065 C. It has been found that satisfactory results are obtained by heating the gold body 14 to a temperature of approximately 600650 C. for a period of about 3 seconds while pressure is applied to the body 14. This can conveniently be carried out by applying heat to the body 14 through the tip element 18, which may in turn be heated in any suitable manner.

During the operation of applying heat and pressure to the body 14, the metal of the body is in a highly malleable or semiplastic state. As a result, under pressure the metal closely envelops or entraps the lamp filament l0 and an intimate, large area contact is created between the body 14 and the filament 10. Simultaneously, a thermocompression bond is created between the body 14 and the surface of the lead wire 12. More specifically, under conditions of heat and pressure diffusion of the materials at the joint interface of the body 14 and the lead wire 12 takes place to produce a strong and highly conductive intermetallic or metallurgical bond. As a result, the mechanical connection between the filament l and the lead wire 12 is firm and reliable. In addition, the electrical resistance at the interconnection is low.

One important advantage of the present invention is that little or no oxide is produced during the operation of terminating the lamp filament. In some known methods of terminating a lamp filament metallic oxides may be produced due to the use of certain materials and/or as a result of the degree of heat required to complete the termination. When heated to elevated temperatures during subsequent use, such oxides may result in contamination of the vacuum within the lamp and/or darkening of the transparent faceplate of the lamp. Since the temperatures required in making the structure are relatively low, and since the structure makes use of gold in a pure form, the problems resulting from the production of oxides are substantially eliminated.

Having reference now to FIG. 4, after the bonding of the body 14 to the surface of the lead wire 12 together with the entrapping of the filament has been completed, the tip member 18 is moved away from the body 14 in order to expose a segment of the metal wire 16. The wire is then severed above the body 14 in any suitable manner. In the preferred arrangement, the wire 16 is melted so that the severing of the wire and the formation of a new body of metal for a subsequent termination operation can be accomplished simultaneously. As illustrated in FIG. 4, a source of heat such as a flame torch 22 is applied to the wire 16 so that the wire melts and is severed. In addition, the melted metal at the end of the wire 16 is formed by the application of heat into a bead or ball serving as a metallic body such as the body 14 illustrated in FIG. ll.

When the wire 16 is severed from body 14, a short length or trail of metal remains. As shown in FIG. 4, the trail may have a thin portion designated as 24 produced as a result of pulling the wire 16 while the metal on the body 14 remains in a plastic state. In order to reduce the profile of the connection between the filament l0 and the lead wire 12, the trail may be removed from the body 14 by breaking it off.

A significant advantage of the method described above is that it may conveniently and economically be carried out with commercially available apparatus normally used for an unrelated purpose. For example, the method may be carried out with thermocompression bonding equipment available from Wells Electronics, Inc. of South Bend, Ind. This equipment is ordinarily used for providing so-called nailhead or ball bonds of a metal wire to a metal receiving surface such as a terminal of a microcircuit or the like, as described in an article entitled Microjoining Processes For Electronic Packaging, Part3 by Ray B. Larson (Assembly Engineering, Nov. 1966).

Although the present invention has been described with reference to details of one particular embodiment, it should be understood that many alterations and modifications may be made by those skilled in the art without departing from the spirit and scope of the invention. Details described here should not be taken to limit the invention except as set forth in the following claims.

What is claimed and desired to be secured by Letters Patent of the United States is:

l. A lamp filament termination structure comprising an electrically conductive lead wire having at least a surface area of gold, a metallic wire filament having a portion located adjacent said area of the lead wire, and a body of gold intimately entrapping said portion of said filament, said body of gold and said surface area of said lead wire being bonded to one another by an intermetallic diffusion bond.

2. The lamp filament structure of claim 1, said surface portion comprising the end of said lead wire.

3. The lamp filament structure of claim I, said metallic wire filament having a spiral configuration and being crushed against said surface area.

4. The lamp filament structure of claim 3, said surface portion comprising the end of said lead wire. 

2. The lamp filament structure of claim 1, said surface portion comprising the end of said lead wire.
 3. The lamp filament structure of claim 1, said metallic wire filament having a spiral configuration and being crushed against said surface area.
 4. The lamp filament structure of claim 3, said surface portion comprising the end of said lead wire. 